Furnaces used in the basic oxygen steelmaking process are usually provided with a refractory lining which frequently consists of bricks formed from magnesia, or dolomite, or mixtures of magnesia and dolomite. These refractory bricks can be used in a burned or unburned condition and are often tar impregnated or tar-bonded. Tar-bonded bricks are generally made by first mixing refractory grains of dolomite or magnesia with pitch or tar and then pressing the mixture to shape, and are usually used in an unburned or unfired condition. On the other hand, tar-impregnated refractory bricks are generally prepared by mixing refractory magnesia particles with a suitable binder, pressing the mixture to shape, drying the shape, firing the refractory shape, and then impregnating the fired shape with pitch.
Refractory magnesia (MgO) is made by "dead burning" the mineral magnesite (MgCO.sub.3), or such magnesium compounds as the hydrate or the chloride, to obtain a residual dense grain of magnesium oxide of stable character. The term dead burning as used in relation to magnesite denotes a procedure in which magnesite is heated to from about 1600.degree. to 2300.degree. C. Dead-burned magnesite is often referred to as "periclase" which denotes a composition having a very high percentage of MgO and which has been processed as by dead burning.
For example, commercially available refractory magnesia now commonly analyzes 96 to 99+ percent MgO, and less than 1.5 percent by weight silica on an oxide basis.
In the past, burned refractory magnesia bricks have usually been produced from dead-burned magnesite (periclase) containing about 96% MgO and having a CaO/SiO.sub.2 ratio ranging from 1.5 to 2.5. Specifically graded size fractions of the periclase, including fine fractions and coarse fractions, are mixed together in conventional mixing equipment such as a muller mixer to provide a dry mix which will produce an optimum packed density. An appropriate binder composition is then added in predetermined proportions to the dry mix and is blended or tempered with the mix to wet out all the grains and provide an easily pressed mixture. These binder compositions usually consist of small amounts of water and a binder material or materials. Typical binder materials that have been used for producing tar-impregnated brick include lignosulfonates, magnesium sulphate, sulfuric acid, dextrin, and the like, with lignosulfonates being generally preferred.
The blended mixture of preiclase and binder is pressed in a mold by a mechanical or hydraulic press under a pressure in excess of 5,000 p.s.i. and preferably about 10,000 to 20,000 p.s.i. This pressed or molded shape is known as a green brick and typically ranges in length from 18 to 27 inches. The green brick is then dried in a suitable manner, such as for example, in an oven at a temperature in the range of about 110.degree. to 204.degree. C. and preferably about 121.degree. to 177.degree. C. to allow the lignosulfonate bond to harden. After mixing, pressing and drying, the refractory shapes are fired in a kiln at maturing temperatures usually in excess of at least about 1538.degree. C. Generally and preferably, such firing will be conducted at a maturing temperature in the range of about 1593.degree. to 1760.degree. C. After firing, the brick is impregnated with pitch under vacuum at about 232.degree. C. The pitch-impregnated brick usually contains about 5 to 6% pitch, of which about 2.1 to 2.6% is retained as carbon after coking.
Refractory linings employed in basic oxygen process steelmaking furnaces must have sufficient strength to support the charge of molten metal in the vessel. Tar-impregnated magnesia refractory bricks produced by the conventional method just described, however, typically have a density of about 2.93 to 3.02 g/cc and a hot modulus of rupture at 1482.degree. C. of about 1000 p.s.i. Bricks of greater strength are desired for use in basic oxygen process steelmaking furnaces. The prior art has employed a variety of techniques, such as refinements in composition and mineral placement, to produce refractory magnesia bricks having a hot modulus of rupture at 1482.degree. C. of between about 1500 to 2400 p.s.i. For example, as disclosed in U.S. patent application Ser. No. 16,237 by Treffner and Filer, filed Mar. 3, 1970, now abandoned, and assigned to the same assignee as the present invention, an improved modulus of rupture at 1482.degree. C. can be obtained by adjusting the CaO/SiO.sub.2 ratio of the fine fractions to 1.6 - 2.1 while the CaO/SiO.sub.2 ratio of the coarse fractions is preferably maintained at 2.0 or higher. The 1482.degree. C. modulus of rupture of brick produced by this method is about 2000 p.s.i., but still greater improvements would be extremely useful in providing refractory linings that can better withstand the conditions that are encountered in the basic oxygen steelmaking furnaces.
The green strength of commercially made lignosulfonate-water bonded periclase brick, that is, the transverse strength of the brick after it is pressed but before it is dried or fired, is relatively low, having a room temperature modulus of rupture of between 10-15 p.s.i. This relatively low green strength causes a serious problem in the handling of shapes longer than 20 inches because when these shapes are lifted by their end, the transverse stresses in the brick generated by their own weight are often sufficient to break the shape in half. Accordingly, a large amount of green brick breakage usually occurs when producing bricks of over 20 inches in length, and a substantial amount of breakage occurs even with smaller bricks. Minimum off-press transverse strength required to prevent breakage has been calculated for shapes with varying lengths as follows:
Minimum M.O.R.(p.s.i.) of Green Brick Required Largest Dimension of Brick for Safe Handling ______________________________________ 15" 6.5 18" 9.0 20" 13.5 24" 17.0 27" 23.0 ______________________________________
Accordingly, it would be desirable to improve the green strength of periclase brick to decrease breakage and lower manufacturing cost while at the same time producing a green brick which can be fired and provide a high hot modulus of rupture.